I'm a RADAR tech with FAA and land-base Enroute RADAR can't track the entire flight so who ever did this knew there was a hole in the RADAR coverage that they could escape through.
The co-pilot had a simulator at his house. Maybe a search of the computer can tell what kind of simulations he was practicing, like maybe low-level flight under the RADAR.
The morons who hi-jacked the 9/11 planes practiced for up to 5 years before they acted.
Almost sounds like a military operation... waaaaay past the 'Atta' level of competence.